This invention relates to microimaging and more specifically to photolithography using lens arrays.
Projection of microimages is common for the manufacturing processes of microdevices including both electric and mechanical microdevices. These include, but are not limited to: electronic type devices such as micro integrated circuits, flat panel display devices, liquid crystal display devices, plasma display devices and field emission display devices; and, and physical devices such as surface acoustic wave devices, micromotors and other micromechanical devices.
Modern microlithography processes sometimes require projection of light images having features as small as, or even smaller than one micrometer (.mu.m). Images for microlithography applications having such small features are generally produced with large lenses in photolithography projection machines. It is a necessary result of physics that the smaller the features of an image, the larger the lens required to faithfully reproduce those small features. However, many problems arise as lens size increase including lens aberrations, thermal stability issues, limited field-of-view, and uniformity issues.
A popular exposure tool for projecting images for photolithographic applications is called a stepper machine. A stepper machine generally includes a very precise large lens with a high numeric aperture. Some stepper machines are capable of faithfully reproducing images with features as small as 0.5 microns. Aberrations limit the useful field-of-view of stepper machines to a circular area of a few centimeters in diameter.
Many devices, although being comprised of very tiny elements, necessarily extend over several centimeters in their entirety. For example, a flat panel display (FPD) device may be fifty centimeters on a side being made up of millions of individual pixels that are only fifty microns on a side. To manufacture devices that require images larger than the maximum field size of a given stepper machine, several stepper image fields are projected in sequential exposures immediately next to each other. This method requires a displacement of the stepper lens with respect to the device substrate that is being printed, and therefore requires very sophisticated motion and alignment equipment. The exposure steps and move steps are repeated until the entire surface of a device substrate is exposed. In this way, a large area device can be "built-up" with a multiplicity of exposures of a single, area-limited stepper field.
It is very difficult to align two stepper fields together. The alignment accuracy is sometimes required to be a small fraction of the image feature size; as small as 100 nanometers. Even with perfect alignment, adjacent images do not always "communicate" well with each other. This is mainly due to third order aberrations such as "pincushion" distortion. Pincushion distortion gets worse as a function of the cube of the radius of an image point as measured from the lens axis in the image plane. These aberrations occur over the entire field area as geometric image placement errors which further complicate alignment of one field to an adjacent field.
Because the field size of a stepper exposure machine is limited, and the alignment of one field to an adjacent field is extremely difficult, it necessarily takes a long time for devices requiring large area exposures to be built-up from a multiplicity of smaller sub-fields. The time that it takes to perform the process limits the amount of devices that a given machine can produce. This limit is expressed as system "throughput" and a primary disadvantage of stepper machines is their low throughput.
It is a further problem in the manufacture of flat panel displays to realize high yield. Sometimes during the manufacture of a device flaws occur that can have a catastrophic effect on the performance of the device. A single error caused by field misalignment, which is common in stepping methods, can cause an entire device to be useless. Nevertheless, the "step-and-repeat" method is the preferred microlithography method for producing large devices like FPDs. The use of stepper machines results in unacceptably low throughput and yield problems.
Another well known method of photolithography for very large areas is called "contact printing". Contact printing requires printing using a photomask which is maintained in very close proximity to the substrate being printed. While contact printing does not suffer from field size limitations, contact printing has an extensive contamination problem. Contamination (or other damage) results from contact between the photomask and the substrate necessitating the frequent and costly replacement of the photomask. Contact printing therefore is not considered a desirable alternative.